Cooperative Interactions and Environmental Control in the Intertidal Clustering of Nerita Textilis (Gastropoda; Prosobranchia)

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1. Somalian populations of the intertidal gastropod Nerita textilis Gmelin show a vertical separation between resting (mid-upper eulittoral) and feeding (lower eulittoral) zone. During high tides and mostly diurnal low tides some snails remain scattered (SF), while other form aggregations (AF) at lower levels of the rocky shore. During the mostly nocturnal low tides both SF and AF migrate downward to feed on microalgae. 2. Lower and upper aggregations are mostly frequented around neap (NT) and spring tides (ST), respectively. Clustering significantly reduces the exposure to waves during high tide, and probably overheating and dehydration during diurnal low tide. 3. About 90 percent of the total population enters into the cluster system throughout each NT-ST-NT cycle. Clustering begins to increase at NT and falls at ST, reaching a maximum (52 percent of the population) between NT and ST. The relative frequency of transition between SF and AF shows a spike after NT and falls after ST. The inverse transition (AF to SF) is negatively correlated to the amount of AF. 4. Return to the original cluster after one feeding excursion is high (76 percent) and decreases slightly at increasing intervals (about 40 percent after 14 feeding migrations), but it becomes significantly lower after one or more rest phases spent in SF. 5. The field study suggests that the periodical (NT) increase in density at the lower rest-zone (tide-pools belt), long-lasting chemical marking of collective rest sites and releasing mucus trails during feeding excursions cause the clustering. According to this hypothesis, a primer group stops at each aggregation site at NT, mostly returning to it by self-trailing after each feeding migration, while the progressive recruitment of snails from SF is caused by the interindividual trail-following during the return migration from the feeding ground. 8. The same model can explain the clustering behaviour of other intertidal gastropods, though the importance of the different parameters (periodical variation in zonation, marking of rest places, trail-following) probably vary among the different species.